1. Field of the Invention
The present invention relates to an electronic endoscopic apparatus. Particularly, the present invention relates to image processing performed on an image obtained by endoscopic photography.
2. Description of the Related Art
An electronic endoscope obtains an image of an object to be observed (for example, stomach wall) by illuminating the object to be observed with illumination light emitted from the leading edge of the electronic endoscope and by receiving reflection light from the object to be observed at an imaging device (for example, CCD (charge coupled device)) that is mounted on the leading edge of the electronic endoscope. Since endoscopic photography is performed in an enclosed dark space, it is necessary that the illumination light is sufficiently light to obtain an image appropriate for diagnosis. However, if the illumination light is too light, the intensity of reflection light from the object to be observed or a tool (for example, forceps) becomes high, thereby a part of the image becoming white. Consequently, an image of the object to be observed corresponding to the white portion is not obtained (so-called halation).
Methods for suppressing halation have been proposed to solve the aforementioned problem (for example, Japanese Unexamined Patent Publication No. 2003-250761 and U.S. Pat. No. 6,545,703). In these methods, halation is suppressed by detecting the halation based on an average brightness value calculated from the pixel value of each pixel of an imaging device or a density histogram and by reducing the light amount of illumination light by adjusting the aperture diaphragm (iris) of lighting (illumination light).
In these methods, it is possible to surely suppress halation. However, when it is necessary to significantly reduce the light amount to suppress the halation, an obtained image becomes too dark and unclear. Therefore, an image appropriate for diagnosis is not always obtained by using these methods.
Meanwhile, even if halation is present in an image, the image is appropriate for diagnosis in some cases. For example, even if a spot-shaped halation is generated only at one point of the image due to some reasons (such as mirror reflection by a drop of water), if the lightness of the image in the area other than the point is appropriate, the image is recognized as an image appropriate for diagnosis. In other words, elimination of halation and obtainment of an image appropriate for diagnosis are not always identical with each other.